Sulfur-Resistant Exhaust Gas Aftertreatment System For The Reduction Of Nitrogen Oxides

ABSTRACT

An arrangement for aftertreatment of exhaust gas for lean-burn internal combustion engines such as diesel engines and Otto engines with direct injection has a NO x  storage catalyzer installed in the exhaust gas train for reducing nitrogen oxides and at which nitrogen oxides are stored in lean operating phases and these stored nitrogen oxides are reduced in rich operating phases. At least one molecular sieve which keeps sulfur dioxide away from the at least one NO x  storage catalyzer is arranged upstream of the NO x  storage catalyzer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject matter of the invention is an arrangement for aftertreatmentof exhaust gas in lean-burn internal combustion engines such as dieselengines and Otto engines with direct injection.

2. Description of the Related Art

In order to adhere to the legally prescribed limits on exhaust gas,virtually all lean-burn internal combustion engines have been outfittedin the meantime with catalytic aftertreatment systems such as:

-   -   NO_(x) storage catalyzers    -   SCR catalyzers, or    -   particulate filters.

When a NO_(x) storage catalyzer is used, the combustion changesconstantly between overstoichiometric combustion and substoichiometriccombustion. In the lean operating phases, the nitrogen oxides are storedin the form of nitrates which are reduced to nitrogen in the richoperating phases by means of carbon monoxide and hydrocarbons. Thestorage in the form of nitrate proceeds by way of NO₂ which accumulatesin the form of nitrate on the barium or calcium storage components.

BaCO₃+2NO₂+0.502

Ba(NO₃)₂+CO₂   (1)

The conversion of the stored nitrate takes place subsequently in therich operating phases by means of carbon monoxide, hydrocarbons (HC) andhydrogen on platinum and/or ruthenium as active component(s).

Ba(NO₃)₂+CO

BaCO₃+2NO+O₂   (2)

2CO+2NO

2CO₂+N₂   (3)

HC+2NO+O₂+0.5H₂+CO

H₂O+2CO₂+N₂   (4)

The NO₂ is needed for this process is formed at NO oxidation catalyzers,usually containing platinum, and/or at the platinum-containing NO_(x)storage catalyzers by means of oxygen which is contained in the exhaustgas and formed from the nitrogen monoxide emitted by the engine.

2NO+O₂

2NO₂   (5)

The problem with this NO oxidation is that the maximum NO₂ proportionsthat can be achieved are limited thermodynamically at high temperatures.As a result, in contrast to other exhaust gas catalyzers, the desiredconversions will decrease again at high temperatures after an increaseat low temperatures and there will not be a pronounced plateau-likeconversion maximum.

Particle separators, as they are called, or particulate filters are usedin power plants and in vehicles to minimize fine particles. A typicalarrangement with particle separators for use in vehicles is described,for example, in EP 1 072 765 A1. Arrangements of this kind differ fromthose using particulate filters in that the diameter of the channels inthe particle separator is substantially greater than the diameter of thelargest occurring particle, while the diameter of the filter channels inparticulate filters is in the range of the diameter of the particles.Due to this difference, particulate filters are prone to clogging, whichincreases the exhaust gas back pressure and reduces engine performance.An arrangement and a method with particulate filters are shown in U.S.Pat. No. 4,902,487. A distinguishing feature of the two above-mentionedarrangements and methods consists in that the oxidationcatalyzer—usually a catalyzer with platinum as active material—arrangedupstream of the particle separator or particulate filter oxidizes thenitrogen monoxide in the exhaust gas by means of the residual oxygenthat is also contained to form nitrogen dioxide which is converted inturn in the particle separator or particulate filter with the carbonparticles to form CO, CO₂, N₂ and NO. In this way, a continuous removalof the deposited solids particles is carried out. Accordingly,regeneration cycles that must be carried out uneconomically in otherarrangements are dispensed with.

2 NO₂+C

2 NO+CO₂   (6)

NO₂+C

NO+CO   (7)

2 C+2 NO₂

N₂+2CO₂   (8)

In order to meet future exhaust gas regulations, it will be necessary touse arrangements for reducing nitrogen oxide emissions and arrangementsfor reducing fine particles emissions at the same time.

As was already mentioned, the NO₂ needed for the storage of the nitrogenoxides in NO_(x) storage catalyzers is formed at NO oxidation catalyzersusually containing platinum and/or at the platinum-containing NO_(x)storage catalyzers themselves. In actual engine operation, however,sulfurization of the NO_(x) storage catalyzers due to sulfur containedin the fuel and/or in the engine oil poses a problem. Owing to thecombustion, SO₂ is formed from this sulfur and is oxidized at the NOoxidation catalyzers downstream to form SO₃.

S+O₂

SO₂   (9)

2SO₂+O₂

SO₃   (10)

In this connection, it has been shown that the amount of SO₃ which isformed and the amount of NO₂ which is formed are directly related; thismeans that a catalyzer forming large amounts of NO₂ generates largeamounts of SO₃ at the same time. This SO₃ forms very stable sulfateswith the storage components of the NO_(x) storage catalyzer, which leadsto a considerable decrease in the NO_(x) storage capacity and,therefore, to reduced catalyzer performance. A regeneration of thecatalyzers can be carried out by actively increasing the exhaust gastemperatures to greater than 900° C., but this can result in thermaldamage to the NO_(x) storage catalyzers. Further, the temperatureincrease is usually connected to an increase in fuel consumption.

SUMMARY OF THE INVENTION

Proceeding from the prior art described above, it is an object of theinvention to prevent the deactivation of NO_(x) storage catalyzers dueto sulfur compounds while overcoming the disadvantages of knownarrangements. A further object of the invention is to provide a methodfor the production of the arrangement according to the invention.

The basic idea is to prevent SO₂ and SO₃ from coming into contact withthe active centers and/or the storage components of the NO_(x) storagecatalyzer so as to prevent the deactivation of the catalyzers by theformation of sulfates.

To this end, a molecular sieve is arranged or applied, e.g., as a layer,upstream of and/or on the NO_(x) storage catalyzer. The pores of themolecular sieve through which the reactants from the flow of gas mustdiffuse at the surface of the catalyzer are designed in such a way thatthey are smaller than the molecular diameters of SO₂ and SO₃ but largerthan the molecular diameters of CO, NO, NO₂ and O₂. Accordingly, themolecules necessary for the reaction at the catalyzer can reach thecatalyzer located downstream of the molecular sieve and/or below themolecular sieve, while the SO₂ responsible for the formation of SO₃ canbe kept away from the NO_(x) storage catalyzer by the molecular sieveowing to steric effects. This works because the molecules NO, NO₂ andSO₂ relevant for the formation of NO₂ have diameters of 1.5 Å to 3 Å,while the diameters for SO₂ and SO₃ are in the range of 7 Å, i.e., thepore diameters of the molecular sieve are therefore advantageouslyselected between 3 Å and 6 Å.

Platinum has proven to be a very active component for the oxidation ofNO. Palladium may also be added in order to increase thermal stability.However, since palladium has only a slight NO oxidation activity, the NOconversion of Pt—Pd mixtures decreases as the proportion of palladiumincreases in comparison to simple Pt—NO oxidation catalyzers.

The molecular sieve can be arranged as a molecular sieve layer directlyon the NO_(x) storage catalyzer or on a carrier arranged upstream of theNO_(x) storage catalyzer so as to achieve the necessary stability forthe desired small layer thicknesses in an advantageous manner.

The defined pore diameters of the molecular sieve can be implementedrelatively simply through the use of zeolites. Different latticeconstants, structures and, therefore, pore diameters can be generatedthrough the specific arrangement of AlO₄— and SiO₄-tetrahedra. Further,the use of silicates, metal silicates, aluminates, metal aluminates,silicophosphates, metal silicophosphates, silicoaluminophosphates,aluminophosphates, metal aluminophosphates, and aluminum silicates forthe molecular sieve is advantageous.

It must be taken into account when selecting a suitable type ofmolecular sieve that while the selectivity between SO₂ and the rest ofthe exhaust gas components increases as the pore diameter decreases, thediffusion of NO, NO₂, CO, N₂, CO₂ and O₂ on or from the active centersis made more difficult at the same time, which can impair the NO_(x)conversion. Since the influence of pore diffusion on the conversionsincreases as temperature increases, different pore diameters andtherefore different types of molecular sieve can be used for differentcases of temperature applications. EDI-type molecular sieves haveespecially small pore diameters up to and including 3 Å, while ABW, AEI,AFR, AWW, BIK, CHA, —CLO, KFI, LTA, NAT, PAU, RHO, —ROG and THO typeshave pore diameters up to and including 4 Å. Pore diameters up to andincluding 5 Åare achieved when using AFT, ATT, ATV, BRE, CAS, —CHI, DAC,DDR, GIS, GOO, HEU, JBW, LEV, MON, PHI, WEN and YUG types. Porediameters up to and including 6 Å are achieved when using APC, EAB, EPI,ERI, EUO, FER, LAU, MEL, MER, MFI, MFS, MTT, MTW, NES and TON types

The above-mentioned designations conform to the IZA (internationalZeolite Association) nomenclature.

When the molecular sieve is constructed as a zeolite, particularly smallpore diameters up to and including 3 Å are achieved in edingtonite-typezeolites, while pore diameters up to and including 4 Å are achieved inLi-A, bikitaite, chapazite, cloverite, ZK-5, zeolite A, natrolite,paulingite, roggianite, and thomsonite types. Pore diameters up to andincluding 5 Å are achieved when using brewsterite, chiavennite,dachiardite, gismondine, goosecreekite, heulandite, Na-J, levyne,montesommaite, phillipsite, wenkite and yugawaralite types. Porediameters up to and including 6 Å are achieved in TMA-E, epistilbite,erionite, EU-1, ferrierite, laumontite, ZSM-11, merlionite, ZSM-5,ZSM-57, ZSM-23, ZSM-12, NU-87 and theta-1 types.

AIPO-18 (AEI), AIPO-22 (AWW), AIPO-52 (AFT), AIPO-12-TAMU (ATT), AIPO-25(ATV) and AIPO-C (APC) can be used as aluminophosphates, and SAPO-40(AFR) can be used as silicoaluminophosphate.

The types of molecular sieve, zeolite, aluminophosphate andsilicoaluminophosphate mentioned above can advantageously be usedindividually or in any combination as molecular sieve material.

The average layer thickness of the molecular sieve or molecular sievelayer should be at least 3 Å. Since the molecules CO, NO, NO₂ and O₂necessary for NO_(x) storage must first diffuse through the molecularsieve layer, the NO_(x) conversions may be limited especially at hightemperatures due to pore diffusion. Therefore, the average thickness ofthe molecular sieve or molecular sieve layer should not exceed 5 μm sothat the influence of pore diffusion is not increased unnecessarily. Incontrast, the layer thickness of the catalyzer layer in a catalyzerrealized by means of coating or extrusion is usually between 5 μm and500 μm.

To improve the conversion at the NO_(x) storage catalyzer, it is usefulto integrate the metals, such as platinum, rhodium, ruthenium, barium,calcium, and palladium, acting as active components in a zeolite matrix,particularly the MFI and/or BEA and/or FAU type(s). However, it must beensured that the molecular sieve or molecular sieve layer does not haveany components, particularly platinum, generating SO₃ or at least has asmaller amount of such components than the actual NO_(x) storagecatalyzer because NO oxidation and SO₂ oxidation usually take place inparallel.

The production of zeolite-containing catalyzers is described in U.S.Pat. No. 5,017,538, U.S. Pat. No. 4,999,173 and U.S. Pat. No. 4,170,571and is therefore familiar to the person skilled in the art.

The zeolite type of the catalyzer and of the molecular sieve ormolecular sieve layer may be identical or different depending on theapplication.

The catalyzers can be produced by extrusion or by coating a ceramic ormetal substrate. Extrusion usually results in honeycomb catalyzers withparallel flow channels (U.S. Pat. No. 3,824,196), whereas with metalcatalyzer substrates the shape and orientation of the flow channels canbe freely selected to a great extent. After the drying and/orcalcination of the catalyzers, the molecular sieve layer is formed inanother work step. Two different methods may be chosen for this purpose.

For one, it is possible to arrange a molecular sieve layer in a mannersimilar to the coating of substrates with a catalyzer washcoat. Thislayer must then be dried and calcinated similar to the catalyzers toensure a stable and solid connection between the molecular sieve layerand catalyzer. When zeolites are used for the molecular sieve layer, thecoating can be carried out by means of a zeolite-containing suspension.The average thickness of a layer applied in this way is usually in therange of 0.5 μm to 5 μm.

When large amounts of hydrocarbons are contained in the exhaust gas,they can deposit on the molecular sieve layer and/or in the pores of themolecular sieve layer, cause coking and lead to clogging of the sievelayer. For this reason it is useful to integrate additional activecomponents in the molecular sieve layer which enable oxidation ofhydrocarbons. These active components include palladium, ruthenium,iridium, rhodium, tungsten, titanium, lanthanum, molybdenum, cerium,and/or manganese. When the elements are integrated in a zeolitestructure, the sublimation temperatures are increased considerably owingto the high steam pressures within the pore structure, so that it iseven possible to use vanadium in the molecular sieve layer up to 750° C.

Another possibility for preventing coking of the molecular sieve poresis to arrange a catalyzer for oxidation of hydrocarbons upstream of themolecular sieve and/or on the side of the molecular sieve facing thehydrocarbon-containing exhaust gas. As was already described referringto the molecular sieve, it can be arranged on the molecular sieve as acatalyzer layer in another work step. Active components includepalladium, ruthenium, rhodium, iridium, tungsten, titanium, lanthanum,molybdenum, cerium, and/or manganese.

It should be noted that when using a molecular sieve through whichhydrocarbons cannot pass, possibly in connection with a catalyzer forthe oxidation of hydrocarbons on the side of the molecular sieve facingthe exhaust gas or upstream of the NO_(x) storage catalyzer, the nitratestored in the NO_(x) storage catalyzer can only be converted in thereaction according to equation (3) but not according to equation (4). Aswas mentioned, the conversion takes place in rich operating phases,where sufficient amounts of carbon monoxide are available for thereduction with a lack of oxygen.

Another possibility for forming the molecular sieve layer, particularlywhen using zeolite-containing, exchanged catalyzers, is to substitutethe concentration of at least one metal at the catalyzer surface with adifferent ion having only a low SO₃— forming activity, or none at all,through ion exchange. In the simplest case, this is accomplished byintroducing an acidic fluid. In so doing, the metal ions in the zeolitestructure are replaced by protons from the acidic fluid.

Since hydrogen-containing zeolites have low stability, the protonsshould subsequently be exchanged for metal cations with a lowSO₃-forming activity and/or a high hydrocarbon-oxidizing activity. Aswas already described above, the elements palladium, ruthenium, iridium,rhodium, tungsten, titanium, lanthanum, molybdenum, cerium, manganese,or vanadium can be used for this purpose. Further, nonmetal cations canalso be used. Depending on the affinity of these elements, theintermediate step of protonization can be omitted and the metals can bedirectly exchanged through selection of a suitable pH. Extremely thinconstructions are made possible by means of molecular sieve layersproduced through ion exchange. Their average layer thickness is usuallybetween 3 Å and 1 μm.

Another possibility for reducing the constructional space of the exhaustgas aftertreatment components is to coat the particulate filter withcatalyzer material and provide it with a molecular sieve layer, forexample, by impregnation, ion exchange or coating. However, it must beensured that free flow channels are still available inside theparticulate filter after coating in order to avoid unnecessarily highexhaust gas back pressure. Therefore, the exhaust gas flow within thefilter structure should flow past, not through, the molecular sievepores so that the reactants only penetrate into the molecular sievepores by means of diffusion processes analogous to the processes atcatalyzer substrates described above and accordingly reach theunderlying catalyzer layer. This can be achieved, for example, by a highporosity of the catalyzer layer. A high porosity of this kind can begenerated, for example, by mixing filler into the washcoat, which fillerevaporates during calcination, or by mixing in zeolite-free,highly-porous washcoat additives or zeolite types with a high porosityand/or large pore diameter. Another possibility with respect to highlyporous filter substrates would be to carry out a thin coating of filtermaterial with the catalyzer material that does not completely cover orclose the filter substrate so that flow channels that are stillsufficiently free remain inside the filter substrate. When the molecularsieve layer is formed subsequently, it must be ensured that thismolecular sieve layer does not close the free flow channels but onlylies upon the freely accessible surfaces of the catalyzer layer as athin layer. This can be carried out in an advantageous manner inzeolite-containing catalyzers by means of the ion exchange at thesurface of the catalyzer as was already described above. As was alsoalready stated, it must be ensured that the pore diameters of themolecular sieve are selected in such a way that the large SO₂ moleculescannot pass the molecular sieve layer, but the smaller, harmless exhaustgas constituents like oxygen, nitrogen, carbon dioxide, nitrogen oxides,water, and carbon monoxide may pass through. If it is not possible toproduce the molecular sieve layer on the catalyzer by pure ion exchange,the catalyzer can be coated with molecular sieve material separately.

The particulate filter on which the catalyzer layer is arranged with themolecular sieve layer arranged thereon can advantageously be made ofsintered metal and/or ceramic and/or filter foam and/or ceramic fibersand/or quartz fibers and/or glass fibers and/or silicon carbide and/oraluminum titanate.

The solution described above presents a particularly economical,space-saving and, therefore, advantageous possibility for a durablecombination of the molecular sieve layer and the NO_(x) storagecatalyzer layer and/or the catalyzer layer for oxidation of hydrocarbonson a catalyzer substrate and/or on a particulate filter so that theycannot be separated from one another without being destroyed.

Naturally, it is also possible to arrange the molecular sieve, NO_(x)storage catalyzer and the catalyzer for oxidation of hydrocarbons onseparate structural component parts with all of the exhaust gas beingguided through the pores of the molecular sieve. However, this wouldresult in an appreciably higher exhaust gas back pressure than is thecase in the solutions described above. When the molecular sieve isarranged separately upstream of the NO_(x) storage catalyzer, it must beensured that the exhaust gas is free from solid particles such as soot,for example, because otherwise the molecular sieve pores would becomeclogged. This is achieved, for example, by arranging the molecular sievedownstream of a particulate filter and upstream of the NO_(x) storagecatalyzer. In an arrangement of this kind, the molecular sieve can alsobe combined in an advantageous manner with the particulate filter inthat the molecular sieve is arranged as a layer on the outlet side ofthe particulate filter so that the molecular sieve and the particulatefilter form a unit that cannot be separated without being destroyed. Inthis case, the coating is carried out in such a way that the pores ofthe particulate filter on its outlet side are completely closed by themolecular sieve so that the exhaust gas must flow through the pores ofthe molecular sieve. However, for this purpose, especially with highconcentrations of hydrocarbons in the exhaust gas, it is usuallynecessary that the molecular sieve have a hydrocarbon oxidation activityand/or that a catalyzer for oxidation of hydrocarbons be arrangedupstream of the molecular sieve and/or on its inlet side becauseotherwise the molecular sieve pores would become blocked by unburnedhydrocarbons. As was already described above, possible active componentsinclude palladium, ruthenium, iridium, rhodium, tungsten, titanium,lanthanum, molybdenum, cerium or manganese.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of the disclosure. For a better understanding of the invention, itsoperating advantages, and specific objects attained by its use,reference should be had to the drawing and descriptive matter in whichthere are illustrated and described preferred embodiments of theinvention.

1. Apparatus for after-treatment exhaust gas in the exhaust gas train ofa lean-burn internal combustion engine, the exhaust gas train having aninput and an output, the apparatus comprising: an NO_(x) storagecatalyzer installed in the exhaust gas train, the catalyzer having atleast one storage component for storing nitrogen oxides during leanoperating phases and at least one active component for reducing storednitrogen oxides during rich operating phases; and a molecular sievearranged upstream of the NO_(x) storage catalyzer, the molecular sievepreventing sulfur dioxide from reaching the catalyzer.
 2. The apparatusof claim 1 wherein the molecular sieve is a molecular sieve layer on theNO_(x) storage catalyzer.
 3. The apparatus of claim 1 further comprisinga carrier arranged upstream of the NO_(x) storage catalyzer, themolecular sieve being provided as a molecular sieve layer on thecarrier.
 4. The apparatus of claim 1 wherein the at least one activecomponent contains at least one of platinum, palladium, ruthenium,barium, rhodium, and calcium.
 5. The apparatus of claim 1 wherein the atleast one storage component and the at least one active componet areembedded in a zeolite.
 6. The apparatus of claim 5 wherein the zeoliteis at least one of types MFI, BEA, and FAU.
 7. The apparatus of claim 1wherein the molecular sieve comprises at least one of zeolites,silicates, metal silicates, aluminates, metal aluminates,silicophosphates, metal silicophosphates, silicoaluminophosphates,aluminophosphates, metal aluminophosphates, and aluminum silicates. 8.The apparatus of claim 1 wherein the molecular sieve has latticestructures of at least one of types EDI, ABW, AEI, AFR, AWW, BIK, CHA,—CLO, KFI, LTA, NAT, PAU, RHO, —RON, THO, AFT, ATT, ATV, BRE, CAS, —CHI,DAC, DDR, GIS, GOO, HEU, JBW, LEV, MON, PHI, WEN, YUG, APC, EAB, EPI,ERI, EUO, FER, LAU, MEL, MER, MFI, MFS, MTT, MTW, NES, and TON.
 9. Theapparatus of claim 1 wherein the molecular sieve comprises zeolites ofat least one of types edingtonite, Li-A, bikitaite, chapazite,cloverite, ZK-5, zeolite A, natrolite, paulingite, roggianite,thomsonite, brewsterite, chiavennite, dachiardite, gismondine,goosecreekite, heulandite, Na-J, levyne, montesommaite, phillipsite,wenkite, yugawaralite, TMA-E, epistilbite, erionite, EU-1, ferrierite,laumontite, ZSM-11, merlionite, ZSM-5, ZSM-57, ZSM-23, ZSM-12, NU-87,and theta-1.
 10. The apparatus of claim 1 wherein the molecular sievecomprises aluminophosphates of at least one of types AIPO-18, AIPO-22,AIPO-52, AIPO-12-TAMU, AIPO-25, and AIPO-C.
 11. The apparatus of claim 1wherein the molecular sieve comprises silicoaluminophosphate SAPO-40.12. The apparatus of claim 1 wherein the molecular sieve has oxidationactivity for hydrocarbons.
 13. The apparatus of claim 1 furthercomprising a catalyzer for the oxidation of hydrocarbons arrangedupstream of the molecular sieve.
 14. The apparatus of claim 13 whereinat least one of the molecular sieve and the catalyzer for oxidation ofhydrocarbons contain at least one of palladium, ruthenium, iridium,tungsten, titanium, lanthanum, molybdenum, cerium, manganese, vanadium,and rhodium.
 15. The apparatus of claim 1 wherein the molecular sievehas a concentration of at least one active component for the oxidationof nitrogen monoxide that is lower than the concentration of said atleast one active component in the NO_(x) storage catalyzer.
 16. Theapparatus of claim 15 wherein the catalyzer has a concentration ofplatinum that is higher than the concentration of platinum in themolecular sieve layer.
 17. The apparatus of claim 1 wherein themolecular sieve layer has a concentration of at least one of palladium,ruthenium, iridium, tungsten, titanium, lanthanum, molybdenum, cerium,manganese, vanadium, and rhodium that is higher than the concentrationof at least one of palladium, ruthenium, iridium, tungsten, titanium,lanthanum, molybdenum, cerium, manganese, vanadium, and rhodium in thecatalyzer.
 18. The apparatus of claim 13 wherein the NO_(x) storagecatalyzer, the molecular sieve layer, and the catalyzer for theoxidation of hydrocarbons are fixed to each other and cannot beseparated without being destroyed.
 19. The apparatus of claim 13 furthercomprising a particulate filter, wherein at least one of the NO_(x)storage catalyzer, the molecular sieve layer, and the catalyzer for theoxidation of hydrocarbons are arranged on the particulate filter. 20.The apparatus of claim 1 further comprising a particulate filter havingan outlet side, wherein the molecular sieve layer is arranged on theoutlet side.
 21. The apparatus of claim 1 wherein the molecular sievehas free passages therethrough with a diameter between 3 Å and 6 Å. 22.The apparatus of claim 1 wherein the molecular sieve has a thicknessbetween 3 Å and 5 μm.
 23. A method for producing an apparatus forafter-treatment exhaust gas in the exhaust gas line of a lean-burninternal combustion engine, the method comprising: producing an NO_(x)storage catalyzer by one of extrusion and coating a substrate, theNO_(x) storage catalyzer having at least one storage component forstoring nitrogen oxides during lean operating phases and at least oneactive component for reducing stored nitrogen oxides during richoperating phases; and providing a molecular sieve layer on the NO_(x)storage catalyzer, the molecular sieve layer preventing sulfur dioxidefrom reaching the catalyzer.
 24. The method of claim 24 furthercomprising arranging a catalytic layer on the molecular sieve layer, thecatalytic layer having at least one active component for oxidation ofhydrocarbons.
 25. The method of claim 23 wherein the NO_(x) storagecatalyzer has a surface with a metal-exchanged zeolite material, andwherein the molecular sieve layer is produced through ion-exchange of atleast one active metal in the surface with one of a metal having no SO₂oxidation activity and nonmetallic cations.
 26. The method of claim 23wherein the NO_(x) storage catalyzer has a surface with ametal-exchanged zeolite material, and wherein the molecular sieve layeris produced through ion-exchange of at least one active metal in thesurface with one of a metal having no SO₂ oxidation activity but a highhydrocarbon oxidation activity.